The use of electronics in controlling IC engine operation is well known. This includes electronic engine control (EEC) equipment for controlling engine fuel injection, fuel density compensation, cold start etc., and vehicle mounted diagnostic equipment which monitor and record engine operating parameters to provide a vehicle maintenance data base, or for feedback of engine performance to the EEC.
One such operating parameter is engine timing, defined as the crankshaft angle interval between a cylinder's full combustion point and its piston's top dead center (TDC) position. The crankshaft angle is measured by monitoring rotation of the engine's ring gear mounted on the flywheel assembly. The ring gear is provided with gear teeth by the engine manufacturer to allow the flywheel to be engaged and driven by the starter motor during engine start-up. By monitoring ring gear rotation with a proximity sensor a series of signal pulses are obtained, each coincident with rotation of a gear tooth past the sensor position. The spacing between signal pulses is crankshaft angle.
The sensed crankshaft angle resolution is limited by the number (T.sub.c) of ring gear teeth; the resolution defined as 360.degree./T.sub.c. The number of teeth on the ring gear varies; by engine model, and manufacturer. It may range from 92 to 152 teeth, corresponding to sensed crankshaft angle intervals of from 3.913.degree. to 2.368.degree..